I mentioned the expression billy button as a slang word for 'tailor' in yesterday's post. Some slang dictionaries, eg Jonathon Green's, say that a billy button is a gullible fool, namely someone who agrees to do a task or piece of work, without checking that they will be paid for it. Dictionaries, especially dictionaries of slang, have lots of expressions beginning billy or Billy, since it is often used as a generic name. It is a Scots word, also used in northern English dialects, that means fellow, comrade, mate or companion. It also means, or meant (OED citations are from the 18th century) 'brother'. The feminine version of billy, ie sister, is tittie. The OED says that both these forms are now considered rude.
Billy is also used as a euphemism for the devil in like billy-o and billy-be-damned.
Other 'billy' words that have caught my eye, or that I sometimes hear, are:
Billy born drunk - a habitual drunkard.
Billy No-Mates - someone with no friends. This, according to the OED, became popular when it appeared as a slogan on t-shirts in the 1990s. It is fairly common today still.
Billy Harran's dog - a time-server, or one who befriends whoever they happen to be with at the time (originally Irish).
billy-noodle - a man who firmly believes, despite all evidence to the contrary, that no woman can resist his charms.